BIL 161 – Plant Form and Function Mechanisms of Water Movement

Transpiration is the process of water movement from root to stem to leaf, and its evaporation from openings in aerial structures such as stems and leaves. Although about 97-99% of the water taken up by the roots is transpired directly into the atmosphere without taking part in a plant’s metabolism, that remaining 1-3% is all that is necessary for the plant to maintain its primary metabolic functions, including photosynthesis and respiration.

I. Water Movement: The Basics A solution is a liquid mixture in which a solute is dissolved and uniformly distributed within a solvent. Water is the most important solvent in biological solutions.

The potential energy of either a solvent or solute particle is the energy it contains as determined by its • position relative to other molecules • internal stresses • electrical charge

In an area with a large number of a given particle, those particles have higher collective potential energy than an adjacent area with a smaller number of that same particle. This difference creates a potential energy gradient. Obeying the Second Law of Thermodynamics, the particles will always follow the gradient, moving from the area of relatively high potential to the area of relatively low potential. This is the basis of both diffusion and osmosis. (Figure 1).

Figure 1. Diffusion (1a) is the net movement of a substance from a region of higher concentration (= potential energy) to a region of lower concentration. Osmosis (1b) is the net movement of solvent molecules from an area of relatively high solvent concentration (= potential) to an area of relatively low solvent potential through a selectively permeable membrane.

Water potential (Ψ) is a measure of the free energy of water. • It is expressed in units of pressure called megapascals (MPa). • It can be compared among different systems, but not measured directly. • It peaks at 0.0 MPa in pure water containing no solutes. • It cannot be greater than 0.0 (by definition). Water containing solutes has fewer water molecules per unit volume than pure water. Thus, the water potential of water containing any solutes will always be less than zero.

The more solutes in the water, the lower its water potential. Water molecules will always follow their potential gradient, moving from high to low potential. (Think of water as always tending to move from a relatively "wet" area to a relatively "dry" area.)

II. Water Movement in Plants: The Transport Path In a plant, water moves via osmosis across the selectively permeable barrier formed by the plasma membrane of the cells. Initial entry into the plant body can occur via one of two pathways. • The apoplast is the space between contiguous cell walls. • The symplast is the inner surface of the plant cell plasma membrane. Plasmodesmata (thin strands of cytoplasm connecting adjacent cells via pores in the cell wall) allow the symplasts of adjacent cells to form a continuous path.

Because water molecules are polar, they form hydrogen bonds with other polar substances, making them • cohesive (they stick to each other) • adhesive (they stick to other polar substances)

Water molecules move through plant vascular tissue en masse because they adhere to the walls of the vascular tissue and cohere to each other.

The path of water entry into the root is shown in Figure 1. Water enters the plant through the root epidermis. Water entering via the apoplast travels along the cell walls it meets the endodermis, which blocks interstitial entry. Water must then enter the endodermis cells through membrane channels and travel via the symplast, the living protoplast of the root cells.

Figure 1. The path of water through the root is shown on the bottom of the figure. (from Biology by Campbell, et al., Pearson Education, Inc.)

Water exits the leaves through microscopic openings called stomates. In more derived plants, the stomates are bordered by guard cells that open and close the stomate in response to • time of day • environmental conditions • osmotic state of the plant

But what makes water actually travel through the xylem from root to stomate?

III. Root pressure Root pressure is the force--generated by differences in water potential--that pushes water from the soil into the roots, and up the stem.

Ordinarily, aqueous root cell cytoplasm contains more solutes than does the water contained in soil. Thus, water potential of the root is lower than water potential of soil. Water will follow this gradient from the soil into the root via osmosis. Water potential in stem tissue is lower than that in the root, and even lower in the leaves. Thus, water follows the potential gradient from root to stem to leaf. The force generated by this potential gradient is known as root pressure.

The stump of a recently felled tree or a topped herbaceous plant often will give visual evidence of root pressure by extruding of water from the cut end. Root pressure pushes water upward, a bit like water being pushed through a hose.

Root pressure is capable of pushing water from the roots to an altitude of approximately 11 meters (36 feet).

Hold on. If root pressure can push water up only that high, how does water reach the top of a redwood tree that towers more than 100 meters? This is accomplished by another force: shoot tension.

IV. Shoot tension Shoot tension is the negative pressure (“suction”) generated as water evaporates from the stomates. Shoot tension is generated primarily by transpiration.

As in root pressure, water molecules follow their potential gradient from root to stem to leaf. But the atmosphere, even on a moderately humid day, has vanishingly low water potential. (Figure 2). This draws water molecules out of the stomate space and into the air. The cohesion of the water molecules keeps a steady stream of water vapor exiting the stomates, as long as they are open.

In small-diameter tubes, such as xylem vessels, a column of water has such great power of cohesion that it has a tensile strength approaching that of a steel wire. If a thin, unbroken column of water extends from root to leaf in the xylem cells, water evaporating through the stomates will generate a powerful negative pressure on the entire column of water right down to the root.

At the root, water will be absorbed in response to the stress on the moving water column. This force is primarily responsible for water movement in vascular plants of any appreciable height.

Shoot tension is capable of pulling water from the roots to an altitude of approximately 100 meters (328 feet).

Figure 2. Transpiration. Water molecules form an unbroken chain as they move from root to stomate. Water potential (Ψ) decreases from root to stem to leaf to atmosphere. Water follows the potential gradient until it exits the stomates into the atmosphere, which has extremely low water potential. (source: Biology by Campbell, et al. Pearson Education, Inc.)